MSSQLSession mssql;

bool ConnectMSSql() {
	String cs = "Driver={SQL Server Native Client 11.0};";
	cs << "Server=***;";
	cs << "UID=***;";
	cs << "PWD=***;";
	cs << "Database=***;";

	while(!mssql.Connect(cs))
		if(!ErrorRetryCancel(
			Format("\1Connection to database server failed:\n%s", mssql.GetLastError())
		))
			return false;
		
	SQL = mssql;
	return true;
}

bool ReconnectMSSql() {
	return SQL.GetErrorCode() == 0 && ConnectMSSql(); // is error code right?
}

		bool retry;
		do {
			retry = false;
			try { // set start date
				q & ::Update(ACTIVITIES)(startDtId, newStartDt).Where(ACT_ID == Id);
			}
			catch(SqlExc &e) {
				if(ReconnectMSSql()) retry = true;
				else {
					Exclamation("[* " + DeQtfLf(e) + "]");
					return false;
				}
			}
		} while(retry);

enum DBTYPE {MS, LITE};

class SqlManager {
private:
	static MSSQLSession mssql;
	static Sqlite3Session sqlite;
	static DBTYPE db_type;
	static String ms_con_str, lite_file;
	
public:
	typedef SqlManager CLASSNAME;
	
	SqlManager();
	void InstallErrorHandler();
	void UninstallErrorHandler();
	void AssignSession();
	static bool Connect();
	static bool ErrorHandler(String error, String stmt, int code, const char *scode, Sql::ERRORCLASS clss);
};
	
MSSQLSession SqlManager::mssql;
Sqlite3Session SqlManager::sqlite;
DBTYPE SqlManager::db_type;
String SqlManager::ms_con_str, SqlManager::lite_file;

// comment / uncomment to change database type
#define DBTYPEMS
//#define DBTYPELITE

#ifdef DBTYPEMS
#define SCHEMADIALECT <MSSQL/MSSQLSchema.h>
#endif

#ifdef DBTYPELITE
#define SCHEMADIALECT <plugin/sqlite3/Sqlite3Schema.h>
#endif

#define MODEL <***.sch>

#include <Sql/sch_schema.h>
#include <Sql/sch_header.h>
#include <Sql/sch_source.h>

SqlManager sqlm;

//===========================================

SqlManager::SqlManager() {
	#ifdef DBTYPEMS
	db_type = MS;
	#endif
	
	#ifdef DBTYPELITE
	db_type = LITE;
	#endif

	switch(db_type) {
		case MS:
			ms_con_str = "Driver={SQL Server Native Client 11.0};";
			ms_con_str << "Server=***;";
			ms_con_str << "UID=***;";
			ms_con_str << "PWD=***;";
			ms_con_str << "Database=***;";
			break;
		case LITE:
			lite_file = "***";
			break;
	}
	
	InstallErrorHandler();
}

void SqlManager::InstallErrorHandler() {
	switch(db_type) {
		case MS:
			mssql.InstallErrorHandler(&SqlManager::ErrorHandler); break;
		case LITE:
			sqlite.InstallErrorHandler(&SqlManager::ErrorHandler); break;
	}
}

void SqlManager::UninstallErrorHandler() {
	switch(db_type) {
		case MS:
			mssql.InstallErrorHandler(NULL); break;
		case LITE:
			sqlite.InstallErrorHandler(NULL); break;
	}
}

bool SqlManager::Connect() {
	DUMP("Connect");
	switch(db_type) {
		case MS:
			while(!mssql.Connect(ms_con_str))
				if(mssql.WasError()) return false;

			break;
		case LITE:
			while(!sqlite.Open(lite_file))
				if(sqlite.WasError()) return false;
				
			break;
	}
	return true;
}

void SqlManager::AssignSession() {
	switch(db_type) {
		case MS:
			SQL = mssql; break;
		case LITE:
			SQL = sqlite; break;
	}
}

bool SqlManager::ErrorHandler(String error, String stmt, int code, const char *scode, Sql::ERRORCLASS clss) {
	DUMP("ErrorHandler");
	DUMP(code);

	// code, scode or clss seem to work only for code = 53!
	// Hence for other errors using substring matching in error string
	// server timeout
	if(code == 53)
		return ErrorRetryCancel("[* Cannot connect to database server:]\&" + DeQtf(error));
	
	// unable to complete login
	else if(error.Find("Unable to complete login process") >= 0)
		return ErrorRetryCancel("[* Database server login error:]\&" + DeQtfLf(error));

	// if error, try to reconnect; if successful, rerun query
	else if(error.Find("An existing connection was forcibly closed") >= 0) {
		if(Connect()) {
			SQL.ClearError();
			SQL.Execute(stmt);
			return !SQL.WasError();
		}
		else return false;
	}
	
	// all other errors
	else {
		Exclamation("[* Miscellaneous error:]\&" + DeQtfLf(error));
		if(SQL.GetTransactionLevel()) {
			SQL.Rollback();
		}
		return false;
	}
}
...
GUI_APP_MAIN {
	if(!sqlm.Connect()) return;
	sqlm.AssignSession();
	SQL.GetSession().ThrowOnError(false);
...

		q * Update(ACTIVITIES)(startDtId, newStartDt).Where(ACT_ID == Id);
		if(SQL.WasError()) return false;